(a) Field of the Invention
This invention generally relates to a system and method for detecting and tracking objects orbiting about the Earth. More particularly, but not by way of limitation, it pertains to a system that uses shadows created by objects as they pass in front of the Sun or the Earth's Moon. Still further, the disclosed invention relates generally to a Sun-tracking or Moon-tracking passive space surveillance system; and, more particularly, to a system and method for detecting the presence and bearing of objects in space in response to the occlusion of Sunlight or Moonlight.
(b) Discussion of Known Art
A known approach at detecting space targets is disclosed in U.S. Pat. No. 5,410,143, to Jones titled “Space target detecting and tracking system utilizing starlight occlusion”, which teaches the use of stars at night to detect the presence of space targets. The Jones system uses a catalog of known star locations to detect the presence of an object. With the Jones system, the presence of an object in space is indicated by the occlusion of known stars.
There are three significant disadvantages to the Jones approach which are as follows: (i) it depends upon the successive occlusion of stars for accurate detection; (ii) it places no practical limitations on the distance, or range, of detectable objects; and (iii) it does not reliably discern object size from object velocity.
The first disadvantage of the Jones method is that it depends on the successive occlusion of stars as the basis for its analysis for determining that there is an object somewhere between the Earth and each of the stars being used. In other words the disadvantage to this approach is that the occlusion of one star after the occlusion of another star lacks sufficient information to lead to the conclusion that a single object caused both (or a series of) occlusions.
The second disadvantage of the Jones system is that the distance between Earth and the stars being used is measured in light years. In other words, the distance from Earth to the nearest star (besides the Sun) is about 9,400,000,000,000 km. This great distance clarifies the problems of reliability associated with the use of stars that are visible at night, where there is a very high probability that something may cross somewhere along the line of sight between the Earth and the star, thereby creating a false alarm.
The third disadvantage of the Jones system is that it does not reasonably discriminate object size from object velocity. This is because, scientifically, starlight (stellar energy) consists of an extended array of points, with each point typically subtending an angle of 1/20 arc-sec or less. Therefore, an object passing in front of such a point source would typically totally eclipse it for a moment. In this situation it would be impossible to tell whether the length of time of the object-star eclipse was due to the objects apparent velocity or its size.
Accordingly, there remains a need for a reliable system that can be used to detect and track objects and other debris within the limited distance of 1AU (Astronomical Unit), or 150,000,000 km. Importantly, there remains a need for a reliable system that can be used to determine the size and velocity of detected and tracked objects and other space debris.
These needs can be met by the following described system which utilizes Sun or Moon occlusion. Both the Sun and Moon are astronomically close objects, at distances of 150,000,000 km and 381,000 km respectively. Therefore, observing occlusions of the Sun or Moon will allow the discrimination of space debris or other objects less then 150,000,000 km or 381,000 km from the Earth.
Because the Sun and the full Moon have equal size/distance ratios, both cover an average of 1800 continuous arc seconds of the sky. Therefore, space debris or other object shadows that move in front of the Sun or Moon will only partially eclipse the observed area as they move across these portions of the sky. This means that these object shadows will have clearly discernable size and velocity as they move across such a continuous source of background light as is present in the Sun or Moon.
Besides meeting these needs, the method of space target identification described here, has other distinct differences than the Jones method. Obviously, observation of the Sun and Moon as objects differs significantly from observation of stars in that the Sun and Moon do not require or benefit from association with a stellar catalog. In addition Sunlight observation is a daytime activity, and useful Sunlight observation never requires an image intensifier. The observation of Sunlight also introduces the useful characteristics of observing a continuous source of ultraviolet light which can be shown to markedly improve a telescope's resolving power. Similarly, the Moon, though only reflecting the Sun's light can also represent a continuous light source to an observer, especially in the case of a full Moon.
Therefore, a review of known devices reveals that there remains a need for a reliable system that can be used to monitor the sky for objects and/or debris that is above or enters the Earth's atmosphere,
There remains a need for a system that can be used to passively track objects traveling above or through the Earth's atmosphere by simple continuous observation of either the Sun or Moon, all without the need for operator skill, such as would be required with normal telescopic observation and the like.
There remains a need for a system and method for detecting objects orbiting the Earth or objects that enter the Earth's atmosphere while using light sources within our solar system, such as the Sun as a useful primary source of light, and the Earth's full Moon as a useful secondary source.